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4 How to Use

4.1 LibreJS in action

After installing the add-on, you will see the LibreJS widget in the add-on bar at the top right of the browser window. After loading a page, left-click on the widget to view the deactivated JavaScript code from the page (both on page and external) and, if applicable, the scripts that were accepted.

4.2 Complaint Feature

It is very important to complain when a site has nonfree JavaScript code, especially if it won’t work without that code. LibreJS makes it easy to complain by heuristically finding where to send the complaint.

When nonfree/nontrivial code is detected in a page, LibreJS attempts to find a relevant contact link or email for the website you are visiting. In order to do so, it will attempt to visit a few links from the current page (for instance, a link labeled “contact” on the same domain as the current page, …)

LibreJS detects contact pages, email addresses that are likely to be owned by the maintainer of the site, Twitter and links, and phone numbers.

After LibreJS detects any of the above, a “Complain” tab will appear on the right of your web browser. When you click on this tab, a large panel will appear with contact information. Ideally, at the top you will find the email address of the maintainer, labeled as the “Email you should use”.

When you complain to the website for their nonfree nontrivial JavaScript, provide them with the link to the JavaScript Trap essay so that they can get more information on what the issue is and how they can solve it on their own site.

LibreJS includes a default subject line and body for the complaint email, with a link to the JavaScript Trap essay. This can be configured in the LibreJS add-on preferences in your web browser.

4.3 Options


LibreJS lets you whitelist domain names and subdomains to bypass the regular JavaScript check. This might be useful, for example, if you are running your own code in a local web server. In order to add a whitelisted domain or url, go to Tools >> Add-ons, or press Control + Shift + A. Inside the add-on window, click on Extensions, and in the list, where you see LibreJS, click on the Preferences button. You will see an input field labeled Whitelist. In the field, enter comma-separated domain names. Do not enter the protocol. For instance to whitelist all the pages of and, enter ‘’. To allow all subdomains from, enter: ‘*’. This will match such sites as and

Complaint tab

This specifies whether the complaint tab appears when a site is running nonfree JavaScript.

Display notifications of JavaScript analysis

This option enables an info bar of realtime JavaScript analysis.

Complaint email subject

Configure the default subject used in complaint emails.

Complaint email body

Configure the default body used in complaint emails.

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